A heating, ventilation and air conditioning (HVAC) system commonly employs a controller for governing the operation of at least some part of the HVAC system. For example, a rooftop unit (RTU), which is most often used to condition a commercial building, may employ a controller to activate a compressor or a furnace to cool or warm air and a blower to deliver the cool or warm air to the building beneath. A controller produces one or more output (control) signals based upon one or more input (sense) signals. Some of these input signals are binary (on/off) in nature. Typical input signals indicate, for example, on/off commands from a thermostat, whether or not a motor is running, or whether or not a compressor is operating normally. The controller may poll or scan (the two terms are used synonymously herein) the various input signals to determine their binary states or, alternatively, the various input signals may be provided as interrupts to the controller, in which case their assertion prompts the controller to determine their binary states.